A reference patent document 1 given below is known as an example of an electric optical distance meter. An optical path diagram of the electric optical distance meter disclosed in the reference patent document 1 is shown in FIG. 4.
In the electric optical distance meter, a distance measuring light (light pulse) L emitted from a light source 1 of a laser diode or the like is emitted through a light transmitting optical system composed of a condenser lens 2, an optical fiber 8, a prism 9, an object lens 10 and the like to a corner cube (prism) 11 placed on a measurement point. The light source 1 is connected to an unillustrated modulator, the modulator is connected to an unillustrated reference signal oscillator, and the distance measuring light L is modulated by a reference signal generated by the reference signal oscillator.
The distance measuring light L reflected by the corner cube 11 passes through a light receiving optical system composed of the object lens 10, the prism 9, a optical fiber 8A, a condenser lens 3, and the like, and enters a light receiving device 7 such as APD (avalanche photodiode). Then, the distance measuring light is converted into a distance measuring signal (electric signal) by the light receiving device 7. A phase difference between the distance measuring signal and a reference signal sent from the modulator is measured, and a distance to the corner cube 11 is obtained from the phase difference.
Incidentally, a split prism 4A and an optical path switching device 5 are inserted between the optical fiber 8 together with a light emitting side and the prism 9, and a split prism 4B is inserted between the prism 9 and the optical fiber 8A together with a light receiving side. The distance measuring light L is divided and sent into an external optical path Po going to the corner cube 11 and an internal optical path Pi going from the light transmitting optical system via the inside of the electric optical distance meter to the light receiving optical system by the split prisms 4A and 4B.
As shown in FIG. 5, the optical path switching device 5 blocks or passes the distance measuring light L going to the internal optical path Pi and the external optical path Po by rotating an optical path switching plate 5C having a blocking plate 5B stuck on a transparent disk 5A, to switch between the internal optical path Pi and the external optical path Po. An unillustrated pulse motor rotates the optical path switching plate 5C. An optical path detector 5D detects that the blocking plate 5B is at either position of the internal optical path Pi and the external optical path Po, that is, a shutter position.
When distance measurement by means of a phase difference is made by using the distance measuring light having passed through the internal optical path Pi (hereinafter this distance measuring light is referred to as reference light R), in the same manner as the distance measuring light L having passed through the external optical path Po, an error specific to the electric optical distance meter can be known. In this way, the distance measurement using the distance measuring light L having passed through the external optical path Po and the distance measurement using the reference light R having passed through the internal optical path Pi are repeated alternately a plurality of times, and the electric optical distance meter corrects the error specific to the electric optical distance meter from a distance measured by using the distance measuring light L having passed through the external optical path Po, and can provide an accurate distance to the corner cube 11.
[Patent Document 1] Japanese Published Unexamined Patent Application No. H08-226969